Re: Unaligned accesses in sha1dc

On Fri, Jun 2, 2017 at 1:17 PM, demerphq [off-list ref] wrote:

Most hash function implementations have code like the following
(extracted and reduced from hv_macro.h in perl.git [which only
supports little-endian hash functions]):

Yes.

Please do *not* try to make things overly portable by adding random
memcpy() functions.

Yes, many compilers will do ok with it, and generate the right code
(single load from an unaligned address). But many won't. Gcc
completely screws it up in older versions on ARM, for example.

Dereferencing an unaligned pointer may be "undefined" in some
technical meaning, but it sure as hell isn't undefined in reality, and
compilers that willfully do stupid things should not be catered to
overly. Reality is a lot more important.

And I think gcc can be tweaked to use "movbe" on x86 with the right
magic incantation (ie not just __builtin_bswap32(), but also the
switch to enable the new instructions).  So having code to detect gcc
and using __builtin_bswap32() would indeed be a good thing.

                     Linus